In technologies for measuring distances, for machining of various types, or for other purposes, by using laser light, a short pulsed laser light with a pulse width of several tens to not more than several hundreds of picoseconds may be desirable. For example, in the technology for measuring distances, pulsed light with a narrow pulse width can enhance the accuracy of measuring distances. Known techniques for generating laser light with a narrow pulse width are described in, for example, Japanese Examined Patent Application Publication No. 7-109911 and Japanese Unexamined Patent Applications Laid-Open Nos. 55-107282 and 2002-368329.
FIG. 1 shows a relationship between driving current and output light of a laser diode (LD). As shown in FIG. 1, while driving current is applied to the LD, the LD emits light that has a pulse-like shape at first, but that gradually attenuates to have a constant intensity. This phenomenon is typical in common semiconductor lasers. This oscillation in light intensity, which occurs in the initial stage of the light emission and gradually attenuates, is called “relaxation oscillation”.
To generate light with a short pulse width, as shown in FIG. 2, driving current may be applied for a very short period Δt to allow only emission of the initial pulsed light. However, the period Δt is on the order of several tens to several hundreds of picoseconds, and driving current is difficult to generate only in the period Δt in a simple circuit.
Under these circumstances, the inventors of the present invention applied for patents relating to a device that is configured to emit short pulsed light by using a circuit including a capacitor and a resistor in parallel connection (Japanese Patent Applications Nos. 2014-169427 and 2014-169429). In the technique used in this device, generation of inrush current to a capacitor and subsequent voltage drop at a resistor are used to make pulse-like current corresponding to one pulse flow to an LD momentarily, thereby enabling emission of light with a short pulse width.
Typical LDs vary their characteristics according to temperature. More specifically, even when the light emission setting is the same, the waveform and the peak intensity of the emitted pulsed light vary according to temperature. Temperature can also affect the characteristics of various types of electronic components such as a resistor, a capacitor, and a transistor.
In the distance measuring technique using laser light, the emission intensity and the pulse waveform of measurement pulsed light affect the distance measurement range and the distance measurement accuracy. Such influence of temperature may be avoided by using a thermostatic device, which can constantly maintain the temperatures of an LD and a light source driving circuit to reduce the influence of temperature variation. However, this method requires further consideration because the thermostatic device is a complicated structure with a heater and a Peltier element, and thus, the thermostatic device can be expensive and can consume a great amount of electric power.
The intensity and the waveform of emitted pulsed light are also susceptible to variations in the characteristics of an LD, a resistor, a capacitor, and other electronic components. In response to this problem, use of high precision components, strict selection of components, thorough adjustment in production of the light emitting device, or other measures may be taken, but they can increase production cost. Nevertheless, a finished product with a stabilized performance can still be affected by the temperature variation as described above.
The characteristics of electronic components can vary over time, which also affects the characteristics of the emitted light. This influence is typically eliminated by replacing or readjusting corresponding components. However, replacing or readjusting of the components takes time and has costs.